1. Field of the Invention
This invention relates to the selective deposition of material in a specific area of a vapor deposition system.
2. Description of the Prior Art
In chemical vapor deposition of ceramic materials, gases are introduced into a hot furnace where they react at the walls of the furnace or at the surface of a substrate or mandrel positioned in the furnace to form a solid deposit or coating thereon. In the formation of a coating of a ceramic material such as silicon carbide (SiC), methyltrichlorosilane (CH.sub.3 SiCl.sub.3), hydrogen (H.sub.2) and argon (Ar) gases are introduced into the reaction chamber through stainless steel injectors. Since methyltrichlorosilane (for convenience termed "MTS" hereinafter) is a liquid at room temperature, Ar is bubbled through the MTS and carries MTS vapor above the liquid to the injectors. Unreacted gases are evacuated from the furnace and cleaned in a gas scrubber. Thick deposits of SiC can be made. Typical conditions for the deposition of SiC are:
______________________________________ Substrate Temperature 1300.degree. C. Furnace or reaction chamber pressure 200 torr Partial pressure of gases Ar 68 torr H.sub.2 102 torr MTS 30 torr ______________________________________
A process is disclosed in application Ser. No. 389,248 filed Aug. 3, 1989 by J. S. Goela, M. A. Pickering and R. L. Taylor and assigned to the assignee of the present invention for fabricating, by vapor deposition, lightweight structures out of refractory materials. The methods and lightweight structures disclosed in that application, which application by reference is incorporated herein, involve a core to define the shape and size of each structure. The core is coated with an appropriate deposit, such as silicon carbide (SiC) or silicon (Si), to give the structure strength and stiffness and for bonding thereof to another surface, for example, the surface of a substrate comprising the faceplate of a mirror being fabricated.
In the fabrication of mirrors, graphite may be used to form a mandrel for replicating on a SiC faceplate. One side of the mandrel is optically fabricated, either as flat or as a convex spherical shape. The other side of the mandrel is lapped flat. The lapped side of the mandrel is bonded by means of pillars and graphite cement to a baffle plate in a vapor deposition reactor. The mandrel is then coated with multiple coats of a suspension of carbon in solvent, following which the surface of the mandrel is buffed or polished to make it as shiny as possible without significantly altering its figure. Deposition of SiC on the mandrel is then effected. Without separating the faceplate from the mandrel, the exposed SiC surface may be etched with hot potassium hydroxide (KOH) to improve bonding of graphite to SiC. A lightweight structure core is then fabricated from flat or curved graphite ribs, as disclosed in the aforementioned Goela et al. application for patent. After being bonded together with graphite cement, the lightweight structure core is bonded with graphite cement to the etched SiC surface of the mandrel. SiC is then deposited to enclose the lightweight structure core following which the baffle plate is separated from the baffle pillars. Controlled edging may be performed to remove excess SiC deposit. Using a blade, the interface between the graphite mandrel and the SiC faceplate may then be opened to recover the SiC coated mirror faceplate. The latter is then ready for Si coating.
Selective deposit to confine Si growth to only the front face of the SiC coated faceplate in the fabrication of the mirror surface is very important. This is for the reason that cracks tending to propagate from the wall of the furnace to the mirror faceplate or other substrate and growth on the backside thereof are disadvantageous. In each case, if not prevented, cracking of the deposit on the front side often results, making replication in a chemical vapor deposition system difficult to achieve, and additionally, requiring post deposition machining to separate the substrate-deposit from the deposition fixture.
Efforts in the prior art to overcome the problem of cracks propagating from the wall of the furnace to the mirror faceplate or other substrate have been unsatisfactory, involving reducing the amount of material deposited around a predetermined selected area in order to isolate the area.
A masking method involving a hollow body made of flexible graphite is disclosed in application Ser. No. 403,957 filed Sept. 7, 1989 by J. S. Goela, R. D. Jaworski and R. L. Taylor, now Pat. No. 4,963,393 for preventing backside growth on faceplates in the fabrication of mirrors in a vapor deposition system.